PROJECT SUMMARY/ABSTRACT Spermatogenesis begins in the neonatal mouse testis with the segregation of prospermatogonia into distinct undifferentiated and differentiating populations. A proportion of undifferentiated spermatogonia retain stem cell potential (as foundational spermatogonial stem cells, or SSCs), and the remainder becomes progenitor spermatogonia that proliferate and differentiate in response to retinoic acid (RA). This initial fate decision is critical, as imbalances cause spermatogenic defects that can lead to human testicular cancer or infertility. It is currently unknown how spermatogonial fate decisions are regulated. We have recently made the exciting discovery that neonatal male germ cells differ in their responsiveness to RA both in vivo and in vitro (in the absence of the niche environment). As early as postnatal day (P) 1, we can identify two distinct subpopulations of prospermatogonia: 1 ? those that cannot respond to RA (?RA-non-responsive?) either in vitro or in vivo, and 2 ? those that can respond but do not yet in vivo (?RA-responsive?). The objective of this proposal is to determine whether the foundational SSC pool forms from prospermatogonia that are intrinsically preprogrammed (predetermination) or from those that happen to occupy a limited number of stem cell niches (selection). Our data strongly suggest that some prospermatogonia are predetermined to remain undifferentiated and become SSCs, and that this fate is manifest by an inability to respond to the proliferation and differentiation signal provided by RA. In the proposed aims, we will explore whether this RA insensitivity is intrinsic (germ cell autonomous) or determined by somatic cells (germ cell non-autonomous), define when during postnatal development differential RA responsiveness appears, and determine whether the foundational SSC pool is formed solely from these RA-insensitive germ cells. In Aim 1, we will assess gene expression heterogeneity associated with neonatal germ cell RA responsiveness. Heterogeneous expression of genes involving RA responsiveness by either the germline or soma could explain which compartment dictates the fate of SSC precursors. In Aim 2, we will define the roles of RA reception and catabolism in RA responsiveness. We predict that RA insensitivity is intrinsically preprogrammed, long lasting, and defines the foundational SSC population. In Aim 3, we will determine whether RA-insensitive prospermatogonia preferentially give rise to foundational SSCs. We will modulate RA levels in vivo and examine the subsequent effect on formation of the foundational SSC pool. Together, the results of these aims will define how RA responsiveness contributes to formation of the foundational SSC pool in the neonatal testis.